1. Field of the Invention
This invention relates to optical couplers More particularly, this invention relates to meniscus lenses to couple excimer beams into optical fibers.
2. Description of Related Art
The ability to couple the beam from a particular excimer laser source into an optical fiber depends on the fluence of the excimer laser beam being less than the fluence damage threshold of the fiber itself. For fused silica fibers, for example, an incident excimer beam of fluence greater than 1000 mJoule/mm.sup.2 may damage the fiber when coupled into the fiber end.
With an ideal excimer laser beam having a uniform fluence across the incident end of the optical fiber, the beam fluence could be simply adjusted to be less than the 1000 mJoule/mm.sup.2 damage threshold of the incident end of the fiber. Actual excimer laser beams, however, do not have a uniformly distributed output, resulting in some portions of the cross sectional output of the excimer beam having very high fluence levels relative to other portions of the excimer beam. These peak fluence levels within the excimer beam may be due to beam nonuniformity in the lasing operation or may be due to the sharpness of focus by a coupling lens used in conjunction with the excimer beam and the optical fiber.
Traditionally, a plano-convex lens was used to couple an excimer laser beam into an optical fiber. The plano-convex lens converges the excimer beam passing through the lens into an infinitesimal area containing substantially all the excimer energy generated by the laser. The laser energy was thus highly concentrated near the lens focal point, creating potentially damaging fluence levels should the incident end of the optical fiber be placed at the lens focal point. Due to the expectation of high fluence levels in the traditional coupling system, the fiber was restricted to remain away from the focal point to prevent fiber damage.
Homogenizing the fluence levels at the incident end of the optical fiber would permit energy coupling with less possibility of fiber damage. Accordingly, an apparatus and method are needed to reduce the peak fluence levels in the excimer beam energy distribution so a fiber capable of accepting a high level of excimer energy may be coupled into an excimer laser beam without being subjected to fluence levels greater than the damage threshold of the fiber as prior couplers allowed.
Goldenberg, U.S. Pat. No. 4,732,448 discloses a laser-fiber coupler with a collimated laser beam entering a plano-convex lens before being directed into a lensed end of a quartz fiber In this disclosure, the plano-convex lens converges the laser energy into a sharp focal point near the fiber such that the cross sectional area of the converged beam is ideally infinitely small at the focal point With the high laser energy directed into a small cross sectional area, the fluence levels at the focal point may far exceed the damage threshold of the fiber To accommodate, Goldenberg teaches that the fiber should be positioned to accept the converged beam at a distance farther from the plano-convex lens than its focal point. At this distance, the incident end of the fiber sees the converged beam as originating from a point source at the focal point. Past the focal point, the laser beam diverges until it reaches the incident end of the fiber, which includes a lens either integral with the fiber or attached to a plane surface of the fiber to recollimate the diverging laser beam within the core of the fiber.
Focusing the beam with a plano-convex lens may cause a sharp focus spot with high enough fluence levels to damage the incident end of the fiber. The laser energy passing through the plano-convex lens is heavily distributed in the central region of the incident laser spot. Since the total energy coupling capability of the fiber is limited by the peak fluence level in the central region of the nonuniform incident laser spot, the higher fluence level in the central region of the incident spot, created by the plano-convex lens coupling, forces the user to move the fiber beyond the focal point such that the focal spot is external to the fiber.